1. Field of the Invention
The present invention relates to an optical disk device, and more particularly to tracking control in an optical disk device which is capable of recording.
2. Description of Related Art
In an optical disk device capable of recording data onto an optical disk, such as a CD-R device, a beam of focused laser light (a laser beam) is irradiated to the optical disk, light reflected from the optical disk is used for generating a tracking error signal, and data is recorded and reproduced while tracking control is conducted based on the tracking error signal. Because when reproducing data a laser beam of reproducing power is irradiated, the reflected light is sampled at predetermined timing to generate a tracking error signal. When recording data, because laser beams of recording power and reproducing power are generated alternately, the reflected light is sampled at the timing of the reproducing power to generate a tracking error signal. In an optical disk device for recording data onto a rewritable optical disk such as a CD-RW, because laser beams of erasing power, recording power, and reproducing power appear sequentially, the reflected light is sampled at the timing of the erasing power to generate a tracking error signal. A tracking error signal can be generated using, for example, the Differential Push Pull (DPP) method.
FIG. 4 shows a relationship between tracks and light spots when a main beam and two sub-beams are focused on an optical disk in the DPP method. The beams are located such that the two sub-beams 102 and 104 sandwich the main beam 100 located in the center. The position of the main beam 100 is shifted from the position of each sub-beam 102 or 104 by half the track pitch in the track width direction. Shown in the drawing is an on-track state, in which the main beam is located on the track n with the sub-beam 102 being located in the middle between the track n and the track n+1 and the sub-beam 104 being located in the middle between the track n and the track n−1. The phases of the sub-beams 102 and 104 are shifted from the phase of the main beam 100, which is used as a reference, by −180° and +180°, respectively. In FIG. 4, each beam spot is divided into two sections in the radial direction, so as to correspond to the configuration of a two-segment detector which receives each light beam reflected from the optical disk and coverts them into an electrical signal.
FIG. 5 shows a configuration of a tracking error signal generating circuit in accordance with the DPP method. Two signals (signals P and N in the drawing) output from a two-segment detector (main detectors) 10 corresponding to the main beam 100 are supplied to a differencer (difference operator) 16, which outputs a push-pull signal corresponding to the main beam 100 (a main push-pull signal). The main push-pull signal (main PP signal) is then supplied to a differencer 24.
On the other hand, two signals output from a two-segment detector (the first detector) 12 corresponding to the sub-beam 102 (the first sub-beam) are supplied to a differencer 18, which outputs a push-pull signal corresponding to the sub-beam 102 (the first sub push-pull signal). Two signals output from a two-segment detector (the second detectors) 14 corresponding to the sub-beam 104 (the second sub-beam) are supplied to a differencer 20, which outputs a push-pull signal corresponding to the sub-beam 104 (the second sub push-pull signal). The first sub push-pull signal (the first sub PP signal) and the second sub push-pull signal (the second sub PP signal) are then supplied to an adder 22. After the two sub PP signals are added by the adder 22, the resultant signal is amplified by K/2 times by an amplifier 23 so as to match the gain of the sub-beams with that of the main beam, and is also output to the differencer 24. Here, K indicates a ratio of the light amount between the main beam and the sub-beam. Assuming that the main beam:sub-beam=10:11, K=10/11, the main PP signal and the sub PP signal are supplied to the differencer 24, which calculates a difference between these signals and an output signal (a DPP signal) from the differencer 24 can be expressed as follows:DPP signal=main PP signal−K/2                (first sub PP signal+second sub PP signal)Because the minor offset caused by the inclination of the optical disk in the radial direction and the shift of an object lens is equally contained in each PP signal, the offset can be removed by calculating the difference as described above.        
After the DPP signal is generated as described above, a predetermined tracking offset signal is added to the DPP signal by an adder 26 and the resultant signal is amplified and then supplied as a tracking error signal to a servo circuit (not shown).
However, the optical axis of a laser beam irradiated from the laser diode (LD) moves depending on the power (more specifically, the temperature), and such a shift of the optical axis further causes the laser spot position on the optical disk to vary between the reproduction time and the recording time. This leads to a problem that accurate tracking cannot be performed during recording, in a structure in which a fixed offset signal is added.
It is of course possible to provide an offset signal for reproduction and an offset signal for recording separately, so that the offset signal for reproduction is added to a DPP signal during data reproduction and the offset signal for recording is added during data recording. This approach, however, does not provide a satisfactory solution when a fixed offset signal is used, because the temperature of the LD is not fixed during recording.
Rather than adding a fixed offset signal for recording to a DPP signal obtained by sampling at the timing of the reproducing power, it is also possible to obtain DPP signals by sampling at the timing of the reproducing power and at the timing of recording power and to generate an offset signal for recording in real time from the difference between these DPP signals. More specifically, it is possible that when recording and tracking data while a main beam and a sub-beam are being irradiated, the reflected component of the sub-beam for tracking control is sampled at the timing of the reproducing power to obtain a push-pull (PP) signal and the reflected light of the sub-beam for tracking control is also sampled at the timing of the recording power to obtain a PP signal, so that an offset signal can be generated in real time from the difference between these two signals. This approach, however, is still disadvantageous in that the circuit scale becomes comparatively large.